Abstract

Abstract

Most previous experiments examining factors affecting the conversion of elemental S into plant-available forms have used indirect methods, such as measuring sulfate accumulation, and have at times resulted in exaggerated estimates of elemental S oxidation rates. Direct measurement of residual elemental S offers a more reliable estimate of conversion rate, but only some of the factors known to affect conversion rate have been investigated by direct methods. A pot experiment was conducted over 238 days to measure conversion rates under a range of elemental S particle sizes (0.1–2.0 mm) and soil textures, and with and without inoculation with sulfur-oxidizing bacteria. Conversion rates were measured by two indirect methods-S uptake by plants and 35SO4 dilution-and one direct method-acetone extraction of remaining elemental S. Initial surface area of elemental S was the prime determinant of conversion rate, but three other factors had to be considered to fully explain observed release characteristics. These were (i) a gradual particle disappearance in 0.1- and 0.2-mm particles, which caused a reduction in conversion rate with time, (ii) a depression in conversion rate for particle sizes ≥ 0.4 mm on noninoculated treatments, and (iii) shape considerations that were imputed to the observed lower conversion rates for the more spherical 2.0-mm size. Three of the four soils-an Arenic Haplustult, a Plinthic Haplustult, and a Typic Torrert-showed increased conversion rates in response to inoculation, with inoculation responses most marked for the larger (> 0.4 mm) particle sizes. The other soil-a Udic Ustochrept-was not responsive to inoculation. No relationship was detected between soil texture and elemental S conversion rate over a range of clay contents from 9 to 52%.

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